Granulizer power feeder with automatic load control



Sept, 15, 1970 O CJKRQLQPP ET AL 3,528,616

GRANULIZER POWER FEEDER WITH AUTOMATIC LOAD CONTROL Filed April 11, 1967 3 Sheets-Sheet 1 if m zz I.

. i W W mu MM r 46 32,, L IL 1 4 45677 A. K471i WW/Aer KW/ r Sept. 15, 1970 O Q KROLOPP ET AL 3,528,616

GRANULIZER POWER FEEDER WITH AUTOMATIC LOAD CONTROL Filed April 11, 196'? 3 Sheets-Sheet 2 Sept. 15, 1970 Q Q KROLQPP ET AL 3,528,616

GRANULIZER POWER FEEDER WITH AUTOMATIC LOAD CONTROL 3 Sheets-Sheet 5 Filed April 11, 1967 O o o WV W g lp i g Q YLT i NQ United States Patent O U.S. Cl. 24134 8 Claims ABSTRACT OF THE DISCLOSURE An automatic load control for a granulizer assembly having a transformer monitoring the current drawn by a grinding roll motor and a load control meter having a low and high current responsive switch adapted to respond to a predetermined low and high current, as monitored by the transformer, and energize either one of two relays which are adapted to cause a transmission motor to be driven in a direction to either increase or decrease the speed output of a variable speed transmission; the variable speed transmission is mechanically interposed between a power feeder motor and a power feeder feed rotor to thereby regulate the speed of the feed rotor and, therefore, the delivery rate of the material to be ground so that the granulizer assembly automatically operates at peak efficiency.

SUMMARY OF THE INVENTION This invention relates to grinding devices such as granulizer assemblies, roller mills and the like; and more particularly to automatic load control means for such a device having a power feeder of the type generally disclosed in copending application Ser. No. 579,634, filed Sept. 15, 1966 and assigned to the assignee of this application, which automatically establishes a condition of maximum grinding efficiency by regulating the delivery rate of the material to be ground to the grinding rolls.

Therefore, a primary object of this invention is to provide automatic load control means to automatically regulate the rate of delivery of material to the grinding rolls of a granulizer assembly to automatically establish a condition of maximum grinding efficiency.

Another object is to provide automatic load control means as above described having time delay circuit means adapted to automatically delay delivery of any material to the grinding rolls until the grinding rolls have come up to speed when the granulizer assembly is turned on and to stop delivery of material to the grinding rolls but maintain power to the grinding roll motor until all material has been discharged from the grinding rolls when the granulizer assembly is turned off, to thereby automatically prevent jamming.

Another object is to provide automatic load control means comprising means for sensing the amount of cur rent drawn by the grinding roll motor and means responsive to the current sensing means to vary the delivery rate of material to the grinding rolls.

Yet, another object is to provide automatic load control means which enables automatic operation of a granulizer at peak efficiency.

A further object is to provide automatic load control means for a granulizer assembly which automatically establishes a condition of maximum capacity on all grinds while maintaining a uniform grind and optimizing grinding roll life and power consumption per pound of material ground.

Other objects and advantages of the invention will become apparent upon reading the following description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated more or less diagrammatically in the accompanying drawings wherein:

FIG. 1 is a perspective view of a granulizer assembly having the automatic load control means of this invention incorporated therein and a suitable control panel therefor;

FIG. 2 is a portion of the schematic diagram of one embodiment of the automatic load control means of this invention; and

FIG. 2a is the remaining portion of the schematic diagram of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Like reference numerals will be used to refer to like parts throughout the following description of the drawings. I Referring now to FIG. 1, a granulizer assembly has been shown as consisting of a base or support 10 having a material discharge end 12 and containing a grinding roll motor 14. The base 10 supports a plurality of vertically stacked sections as at 16, 18 and 20. Each section is provided with opposed grinding rollers, not shown, adapted to grind material such as coffee beans and the like. A power feeder assembly 22 is positioned above the uppermost grinding section 20. The power feeder 22 has a flared delivery structure 24, having an open end 26 through which the material to be ground is fed into the power feeder assembly. The power feeder 22 also has a feed housing 28 containing a fluted feed rotor, not shown, which when rotated, sequentially feeds a metered quantity of the material to be ground uniformly throughout the length of the grinding rolls of section 20 and hence to the grinding rollers of sections 18 and 16.

The grinding sections 16, 18 and 20' and the power feeder 22 are of the type generally illustrated and described in the co-pending application Ser. No. 579,634, filed Sept. 15, 1966 and assigned to the assignee of this application. For details of the grinding sections and power feeder assembly, reference is made to the above application.

The grin-ding roll motor 14 is shown operatively connected to the grinding rollers of sections 16, 18 and 20 by a chain, belt, or other suitable drive, as at 30 and 32 in FIG. 1.

In the particular embodiment of the automatic load control depicted and to be hereinafter described, the feed rotor is driven by a constant speed motor 34 which is operatively connected to the feed rotor through a speed varying device 36 and a suitable driving connection 38. The speed varying device contains a motor 40' adapted to vary the speed of the feed rotor and, thus, the material delivery rate to the grinding rolls, between zero and an upper limit. It should be understood, however, that the automatic load control of this invention may utilize a variable speed motor to vary the speed of the feed rotor.

Further, although the speed varying device 36 has been depicted and will hereinafter be described as a variable speed transmission, it should be understood that other mechanical drive speed varying devices such as a variable pitch pulley or a V-belt drive with a sliding or tilting motor 34 may be used.

A control panel for the granulizer assembly is shown generally at 42 as comprising an ammeter 44 and the following indicator lights and switches: a manual-automatic mode selection switch and indicator light 46a and 46b respectively, a feed rate decrease switch and indicator light 48a and 48b respectively, a neutral indicator light 50, an increase feed rate switch and indicator light 52a and 52b respectively, an alarm indicator light 54, an emergency stop switch and indicator light 56a and 56b respectively, a power-on switch and indicator light 58a and 58b respectively, a power feeder starter switch and indicator light 60a and 60b respectively, a grinding roll motor starter switch and indicator light 62a and 6219 respectively, and a power feed motor starter switch and indicator light 64a and 6412 respectively. The manner in which the above switches and indicator lights may be incorporated in the automatic load control means of this invention and the functions which they may be adapted to effect will be discussed hereinafter. The control panel may also include an hour meter 66.

START-UP CIRCUIT AND OPERATION The schematic diagram of the automatic load control of this invention has been shown in FIGS. 2 and 2a with lines 70, 72 and 74 of FIG. 2a being continuations of lines 70, 72 and 74 respectively of FIG. 2.

The power-on switch 58a and indicator light 58b are shown connected in series to terminations 76 and 78 so that when the switch 58a is closed, the indicator light 5817 may be energized by a power source, such as a 115 V. AC. 60 cycle source, connected between terminations 76 and 78. Accordingly, light 58b will provide a visual indication of the condition of switch 58a.

The grinding roll motor starter switch 62a is shown as having dual, normally open, switch contacts 80 and 82. The switch contacts 80 are connected, in part, in series with switch 58a and a grinding roll motor starter relay 84 having normally open contacts 84a, 84b, 84c and 84d, so that when switch 58a and contacts 80 of switch 6211 are closed, relay 84 is energized and relay contacts 84ad close. Relay contacts 8411- are connected in lines 86, 88 and 90 respectively, which connect the grinding roll motor 14 to a suitable power source having terminations 92, 94 and 96. Accordingly, when switches 58a and 62a are closed, the grinding roll motor 14 is energized. An indicator light 62b may be connected in parallel with starter relay 84 so that the light will be energized when the starter relay is energized, thereby giving a visual indication that the grinding roll motor 14 has been started.

A relay 98, having normally open contacts 98a-d and normally closed contacts 98c, and a time delay relay 100, having normally open contacts 100a, are connected in parallel with relay 84 through switch contacts 82 of switch 62a so that when switches 58a and 62a are closed, relays 98 and 100 will energize, contacts 98a-d and 100a will close, and contacts 98a will open.

Relay contacts 84d and 98a are connected in parallel with the switch contacts 80 and 82 respectively of switch 62a as shown in FIG. 2, so that after momentarily depressing and closing switch 62a, thereby closing contacts 84d and 98a, contacts 80 and 82 may be opened without affecting any circuits previously energized.

The power feeder drive motor 34 is shown connected on one side to termination 76 by line 70 and normally open relay contacts 102a; the other side of motor 34 is connected to termination 78 by line 74 and normally open relay contacts 102]). Relay contacts 2a and 102b are actuated by a power feeder motor starter relay 102 connected in series with the normally open power feeder starter switch 60a and relay contacts 980 and 100a; this series combination is connected in parallel with the grinding roll motor starter relay 84. Accordingly, the power feeder drive motor 34 will energize when switches 58a, 62a and 60a are closed after sufiicient time has elapsed after the closing of switch 62a to enable time delay relay contacts 100a to close.

As previously mentioned, when switches 58a and 62a are closed, the grinding roll motor 14 is energized. It has been found that if material is fed to the grinding rolls before they have come up to running speed, jamming is An indicator light 6012 may be connected in parallel with starter relay 102 so that when it is energized, light b is energized, thereby giving a visual indication that the power feeder drive motor 34 has been started.

AUTOMATIC LOAD CONTROL CIRCUIT AND OPEMT-ION As previously mentioned, the power feeder drive motor 34 is connected to the feed rotor by a variable speed transmission 36 and a suitable driving connection, such as a sprocket and chain or belt drive 38.

As the power feeder feed rotor is driven by the motor 34, the feed rotor delivers a continuous flow of the material to be ground to the grinding rolls. The delivery rate of material may be varied by either increasing or decreasing the rotational speed of the feed rotor. Depending upon factors such as the cubic density and moisture content of the material to be ground and the grind setting of the grinding rolls, there is zone or range of material feed rates consistent with efiicient operation of the granulizer assembly, with efficiency being expressed in terms of the maximum material feed rate on all grind settings which will yield a consistently uniform ground product and optimize the grinding roll life and power consumption per pound of material ground.

It has been found that the eflicient zone or range of material feed rates may be correlated with the current drawn by the grinding roll motor 14. Accordingly, by providing means for sensing the current drawn by the motor 14 and means responsive thereto to automatically regulate the feed rotor speed (material delivery rate), the granulizer assembly may be automatically operated at peak efficiency.

Referring now to FIG. 2a, the transmission motor 40 is shown as having a primary winding 104 connected in 'parallel with the power feeder motor starter relay 102,

so that when the power feeder motor 34 is energized, winding 104 will be energized. The transmission motor 40 has two secondary coil circuits 106 and 108 arranged, so that when circuit 106 is completed, the motor is driven in one direction and when circuit 108 is completed, the motor is driven in the opposite direction. The transmission motor secondary circuit 106 contains normally open relay contacts 110a and normally closed relay contacts 112a; the transmission motor secondary circuit 108 contains normally open relay contacts 1112b.

A transformer is shown generally at 114 in FIG. 2a as having a primary winding 116 connected in series with grinding roll motor power line 86 and a secondary winding 118 connected to a load control meter 120. The load control meter 120 contains the optical ammeter 44, a normally closed low current switch 120a and a normally closed high current switch 12Gb. The load control meter 120 is also shown connected in parallel with the grinding roll motor starter relay 84 so that when the grinding roll motor 14 is energized, the load control meter 120 will be energized. A potentiometer 122 may be connected in series with the transformer secondary winding circuit.

The load control meter 120 is adjusted such that: when the current in winding 118 falls below a predetermined value, contacts 12011 are closed and contacts 12% are open; when the current increases above a predetermined value, contacts 12% are closed and contacts 120a are open; and when the current in Winding 118 is within a zone between the aforementioned high and low current values, contacts 120a and 12012 are open.

The manual-automatic load selection switch 46a is shown in FIG. 2 as having four sets of switch contacts 124, 126, 128 and 130. When the switch is in the automode, contacts 126 and are closed and the contacts 124 and 128 are open.

The automatic mode circuit will first be described.

A relay 110, having normally open contacts 110a in the secondary winding circuit 106 of transmission motor 40 and normally closed contacts 11%, is shown connected in series with a high limit switch 132, switch contacts 126 of switch 460, and the low current switch 120a of the load control meter 120; this series combination is connected in parallel with relay 98 so that when relay 98 is energized, relay 110 may be energized.

A second relay 112 is shown connected in series with a low limit switch 134, switch contacts 130 of switch 46a, the high current switch 12% of the load control meter 120, and contacts 98b of relay 98; this series combination is connected in parallel with relay 98 so that when relay 98 is energized, relay 112 may be energized. Relay 112 contains normally closed contacts 112a in the secondary Winding 106 of transmission motor 40, normally open contacts 112b in the secondary winding 108 of motor 40, and normally closed contacts 1120.

Accordingly, when the motors 14 and 34 have been started by sequentially closing switches 58a, 62a and 60a, should the current in the secondary winding 118 of transformer 114 fall below the low set value, indicating that not enough material is being fed to the grinding rollers and, therefore, that the granulizer is not operating efliciently, switch 120a of load control meter 120 automatically closes and energizes relay 110.- Upon energizing relay 110, relay contacts 110a close, thereby completing the secondary winding circuit 106 of the transmission motor 40. With secondary Winding circuit 106 closed, the motor 40 is driven in a direction increasing the transmission output speed and thus increasing the speed of the rotor and the delivery rate to the grinding rolls of the material to be ground. When the material delivery rate has increased to the point where the current drawn by the grinding roll motor 14 and current in the transformer secondary winding circuit is within the efficient operating zone, contacts 120a open, thereby opening the trans mission motor secondary circuit 106 and disabling the transmission motor 40.

Should any condition change, such as the material moisture content or cubic density, which increases the current draw of the grinding roll motor 14 so that the current in the transformer secondary winding 118 exceeds the high current set point of load control meter 120, the high current switch 12011 closes. The closing of switch 120]), energizes relay 112, thereby closing contacts 112b and opening contacts 112a and 1120. When contacts 11212 are closed, the secondary winding circuit 108 of transmission motor 40 is completed thereby driving the variable transmission 36 in a direction reducing its speed output and thus reducing the speed of the feed rotor and the delivery rate to the grinding rolls of the material to the ground. When the delivery rate of material has been reduced to a point where the current draw of the grinding roll motor 14 is within the efficient operating zone, the load control meter 120 opens contacts 120b, thereby opening secondary circuit 108 and disabling the transmission motor 40.

The neutral or efiicient operation indicator light 50 may ibe connected in series with the normally closed contacts 11% of relay 110, the normally closed contacts 1120 of relay 112, and the normally open contacts 98d of relay 98; this series combination may be connected in parallel with the power feeder motor starter relay 102 so that when starter relay 102 is energized, light 50 may be energized. Accordingly, when the motor 34 is energized and the current within the transformer winding 118 is within the efiicient operating zone (relays 110 and 112 de-energized), indicator light 50 will be energized, thereby providing a visual indication that the material feed rate is constant and the granulizer assembly is operating at peak effiiciency.

Should the supply of unground material to the power feeder assembly 22 be interrupted or stopped, the current drawn by the grinding roll motor 14 would decrease and the transmission motor 40' would futilely drive the transmission 36 in a direction to increase the material feed rate. To provide for this occurrence, the high limit switch 132 may be incorporated in the variable speed transmission 36 and adapted to switch when the transmission output speed reaches a predetermined value. The high limit switch 132 may be connected in series with the low current contacts 120a and relay coil as shown in FIG 2, so that when the transmission output speed reaches the predetermined value, relay 110 is de-energized, thereby disabling the transmission motor 40 and prohibiting any further increase in the transmission output speed. Upon switching and de-energizing relay 110, the high limit switch 132 may be adapated to energize the indicator light 54, thereby giving a visual indication that the delivery of material to the power feeder 22 has been interrupted.

The transmission 36 may also be provided with a low limit switch 134 adapted to switch when the transmission output speed is driven to zero. The switch 134 may be connected in series with the high current switch b and relay 112 so that when the transmission 36 is driven to a condition of zero output speed, relay 112 is de-energized thereby opening the transmission drive motor secondary circuit 108 so that the transmission motor 40 does not further urge the transmission to reduce its output speed. The low limit switch 134 may also be adapted to energize a second time delay relay 136 having normally closed contacts 136a after de-energizing relay 112 to enable sequential shut-down of the motors 14 and 34. The shutdown procedure, circuits, and the circuit operation will be hereinafter described.

MANUAL MODE As previously mentioned, the manual-automatic mode selection switch 460 may be adapted, in conjunction with other circuits, to enable manual operation of the granulizer assembly. One such adaptation is shown in FIG- URES 2 and 2a and is as follows. When the manual switch 460 is in the manual position, switches 124 and 128 are closed and switches 126 and are open. The momentary, manual increase switch 52a may be connected in series with switch contacts 124 of switch 46a; this series combination may then be connected in parallel with the automatic low current switch 120a and switch contacts 126 of switch 460. Accordingly, when the manual-automatic mode selection switch 46a is in the manual position, increase switch 52a provides the manual equivalent of the automatic low current switch 120a of load control meter 120, so that when the manual increase switch 52a is closed, relay coil 110 is energized and the transmission motor 40 is driven in a direction increasing the output speed of the transmission 36 and thus, the delivery rate of the material to be ground.

Similarly, the manual decrease switch 4811 may be connected in series with switch contacts 128 of switch 46a; this series combination may then be connected in parallel with the series combination of switch contacts 130 of switch 46a and the automatic high current switch 12011. Accordingly, when switch 46a is in the manual position, the decrease switch 480 provides the manual equivalent of the automatic high current switch 120k of load control meter 120, so that when the manual decrease switch 48a is closed, relay coil 112 is energized and the transmission motor 40 is driven in a direction reducing the transmission output speed and thus, the delivery rate of the material to be ground.

The optical ammeter 44 of load control meter 120 may be adapted to provide a visual indication of the current being drawn by the grinding roll motor 14 as monitored by the secondary winding 118 of transformer 114. The machine operator may then manually control the rate of delivery of material to the grinding rollers to establish or maintain a condition of maximum grinding efficiency by observing the optical ammeter 44 and closing either switch 52a or 48a to increase or decrease the material delivery rate to maintain the current within the eflicient operating zone.

The manual mode indicator light 46b may be connected to power source terminations 76 and 78 through the switch contacts 128 of switch 46a and power-on switch 58a so that with switch 58a closed, indicator light 4612 will be energized when switch 46a is in the manual position.

The increase and decrease material feed rate indicator lights 52b and 48b respectively may be connected in parallel, respectively, with relays 110 and 112 as shown in FIG. 2 so that when the material feed rate is being increased, light 52b will be energized and when the material feed rate is being decreased, light 48b will be energized.

SHUT-DOWN CIRCUITRY AND OPERATION As shown in FIG. 2, the normally closed, momentary stop switch 64a may be connected in series with the parallel combination of relay "98 and time delay relay 100 so that when stop switch 64a is momentarily opened, relays 98 and 100 are de-energized, relay contacts 98ad and 100a are opened, and relay contacts 98c are closed. The starter relay 102 may be provided with a third set of contacts 1020 which may be connected in parallel with the series combination of contacts 980 and 100a as shown in FIG. 2a, so that when relays 98 and 100 are de-energized, as above, the circuits energized through contacts 980 and 100a will remain energized.

The opening of contacts 98a and 98b disables the low and high current swtiches 120a-b and relay 110.

The normally closed contacts 98c of relay 98 are shown connected in parallel with the series combination of high current switch 12%, switch contacts 130 of switch 46a, and the normally open contacts 98ab of relay 98 so that when relay 98 is de-energized, relay coil 112 will be energized irrespective of the current draw of motor 14. Therefore, when stop switch 64a is opened, the transmission motor 40 will drive the transmission 36 to a zero output speed and thus stop delivery of material to the grinding rolls.

When the transmission 36 has been driven to a zero output, limit switch 134 switches to a position energizing time delay relay 136. Time delay relay 136 has normally closed contacts 136:: which may be connected in series with the grinding roll motor starter relay 84 and the power feeder motor starter relay 102 as shown in FIG. 2a so that when contacts 136a open, the motors 14 and 34 are de-energized. The relay 136 is selected with a time delay sufficient to enable all material to be discharged from the grinding rolls before the motors 14 and 34 are de-energized to thereby eliminate the possibility of jamming when the granulizer assembly is restarted. When relay 84 is de-energized, in addition to opening contacts 84a-c which de-energize motor 14, contacts 84d are opened, thereby de-energizing all circuits and components except the indicator lights 58b and 46b.

The indicator light 64b may be connected in parallel with time delay relay 136 as shown in FIG. 2 so that when the transmission 36 has been driven to a zero output, light 64b will energize, thereby giving a visual indication that delivery of material to the grinding rolls has stopped.

To enable immediate, emergency shut-down of the granulizer assembly, the normally closed, momentary, emergency stop switch 56a may be connected in series with the parallel combination of relay contacts 84d and starter switch 62a as shown in FIG. 2, so that when switch 56a is momentarily opened, all circuits and components are immediately de-energized except indicator lights 58b and 46b.

The emergency stop indicator light 56b may be connected in series with emergency stop switch 56a, as shown in FIGS. 2 and 2a, so that once start-up of the granulizer assembly has been initiated by closing switch 58a and starter switch 62a, light 56b is energized until either sequential shut-down has occurred after opening switch 64a or until the emergency stop switch 56a is opened.

The motor starter relays 84 and 102 may be provided with overload heater relays which have normally closed contacts 138, 140, 142 and 144 which are connected in series with the starter relays as shown in FIG. 2a and are adapted to open at a predetermined temperature to there by de-energize the starter relays when necessary to prevent overloading.

A fuse, such as at 146 may be connected in series with the load control meter 120, as shown in FIG. 2a, to protect this component from damaging current loading.

A suitable resistor, such as at 148, may be connected in series with each indicator light to fix the current level through each light to a suitable level.

The hour meter 66 may be connected in parallel with the grinding roll motor starter relay 84 as shown in FIG. 2a to monitor the total running time of grinding roll motor 14.

Although the load control means of this invention has been described in conjunction with a granulizer assembly of the type adapted to granulize coffee beans and the like, it should be understood that the load control means of this invention may be used on roller mills and other like devices which are adapted to grind or mill a variety of materials.

Further, although a preferred embodiment of the invention has been depicted and described, such embodiment is intended to be exemplary only, and not definitive. It should also be understood that many additions, alterations, and variations may be made without departing from the inventions fundamental theme. Acccordingly, the scope of the invention should be limited only by the scope of the following appended claims.

We claim:

1. A granulizer device for grinding coffee and similar materials including the combination of:

opposed grinding rolls having a plurality of grind settings,

a power feeder having an upper open end through which material to be ground is fed, and a fluted feed rotor, positioned at the opposite end therein and adjacent to the opposed grinding rolls which, when rotated, sequentially feeds a metered quantity of the material to be ground from a location thereabove substantially fixed with respect to the grinding rolls, said rotor being adapted to deliver the material directly to said grinding rolls,

a rotor motor adapted to drive said rotor,

variable speed control means operably connected with said rotor motor,

a grinding roll motor adapted to drive said grinding rolls, and

automatic load control means to operate the rotor motor variable speed control means, said automatic load control means being responsive to grinding roll motor current for controlling the speed of the feed rotor to thereby control the delivery rate of said material through said fluted teed rotor to said grinding rolls to maintain a predetermined condition of maximum grinding efliciency.

2. The structure of claim 1 further characterized by and including first time delay circuit means operable to deactivate said rotor motor for a predetermined time following energizing of the grinding roll motor to automatically prevent jamming when said granulizer device is turned 3. The structure of claim 1 further characterized by and including second time delay circuit means operable to deactivate said grinding roll motor after a predetermined time following de-energizing of the rotor motor, said predetermined time being sufiicient to enable all material to be discharged from said grinding rolls before said grinding roll motor is de-energized, thereby automatically preventing jamming when said granulizer device is shut down.

4. The structure of claim 1 further characterized in that said load control means includes means for sensing the amount of current drawn by said grinding roll motor, and

said speed control means includes means responsive to said sensing means to vary the material delivery rate of said rotor.

5. The structure of claim 4 further characterized in that said power feeder motor is a constant speed motor and said speed control means includes a variable speed transmission adapted to vary the material delivery rate of said rotor.

6. The structure of claim 4 further characterized in that said speed control means is a transmission having a variable output speed and a transmission motor adapted to vary said transmission output speed.

7. The structure of claim 6 further characterized in that said grinding roll motor has circuit means adapted to connect said motor to a power source and said sensing means include a transformer having a primary winding electrically connected in said grinding roll motor power source connecting circuit and a secondary winding electrically connected to current responsive switch means,

said switch means including a first switch responsive to a low current in said transformer secondary winding and a second switch responsive to a high current in said transformer secondary winding, with said first switch adapted to complete a first circuit when the current in said secondary winding reaches said low value and said second switch adapted to complete a second circuit when the current in said secondary winding reaches said high value.

8. The structure of claim 7 further characterized in that said responsive means further includes a first relay having an actuating coil electrically connected in said first circuit and a second relay having an actuating coil electrically connected in said second circuit,

said first and second relay having switch means adapted to interrupt and complete circuits in the transmission motor secondary winding so that when said first circuit is completed, thereby energizing said first relay coil, the transmission motor is driven in a direction increasing the speed of said power feeder and thereby increasing the material delivery rate to said grinding rolls, and when said second circuit is completed, thereby energizing said second relay coil, the transmission motor is driven in a direction decreasing the speed of said power feeder and thereby decreasing the material delivery rate to said grinding rolls.

References Cited UNITED STATES PATENTS 3/ 1923 Gillespie 241225 X 6/1944 Mercer 318-11 12/1949 Adams 241-34 3/1950 Adams 2 41-34 7/ 1951 Alciati 241-222 XR 10/1952 Hailey 241 7/1956 Nauta 24135 9/1960 Williamson 24135 XR 1/ 1964 McCarty 24134 XR 9/1964 Varner 31811 XR 5/1942 Dantzig 241---- 3/1941 Hardinge 24-134 3/1961 Molling 241-30 FOREIGN PATENTS 12/1959 Germany.

9/ 1959 Great Britain.

US. Cl. X.R. 

